The application of corrosion-resistant coatings to metal articles in order to protect the surfaces thereof from degradation by oxidation, galvanic, or other chemical attack is a vastly important field of study. Much effort has been devoted to extending the useful life of articles subject to corrosion by coating the article with a corrosion-resistant composition. Coatings are also applied to substrates for protection against wear. Coatings with corrosion-resistant and wear-resistant properties are applied in many different ways. Typically, metal substrates are coated with corrosion- and wear-resistant coatings by dipping the metal article in a bath of the coating or by the use of an applicator such as a spray nozzle, brush, roller, etc. Chemical vapor deposition, as well as electroplating and electroless-plating, have also been utilized. In accordance with the present invention, a corrosion-resistant and wear-resistant coating is applied to a metal substrate to protect the surfaces of the substrate by a thermal spraying process.
Thermal spray processes are a well known family of coating technologies that include detonation guns, high-velocity oxyfuel spray processes, wire-arc spraying, and both air and vacuum plasma spraying. U.S. Pat. No. 5,451,470 of Ashary et al.; U.S. Pat. No. 5,384,164 of Browning; U.S. Pat. No. 5,271,965 of Browning; U.S. Pat. No. 5,223,332 of Quets; U.S. Pat. No. 5,207,382 of Si et al.; and U.S. Pat. No. 4,694,990 of Karlsson et al. collectively describe thermal spray processes, and are herein incorporated by reference.
Thermal spraying is a process of applying coatings of high performance materials, such as metals, alloys, ceramics, cermets, and carbides, onto more easily worked and cheaper base materials. The purpose of the coating is to provide enhanced surface properties to the cheaper bulk material of which the part is made. Because of its ability to deposit virtually any material (and many combinations of materials), thermal spray has a wide and growing range of applications.
Existing thermal spray processes are compared in Table 1.
TABLE 1Comparison of Thermal Spray TechnologiesFlame powder: Powder feedstock, aspirated into the oxygen/fuel-gasflame, is melted and carried by the flame onto the workpiece.Particle velocity is relatively low, and bond strength of depositsis low. Porosity is high and cohesive strength is low. Spray ratesare usually in the 0.5 to 9 kg/h (1 to 20 lb/h) range. Surfacetemperatures can run quite high.Flame wire: In flame wire spraying, the only function of the flameis to melt the material. A stream of air then disintegrates themolten material and propels it onto the workpiece. Spray rates formaterials such as stainless steel are in the range of 0.5 to 9 kg/h(1 to 20 lb/h). Substrate temperatures are from 95 to 205° C.(200 to 400° F.) because of the excess energy input required forflame melting.Wire arc: Two consumable wire electrodes are fed into the gun, wherethey meet and form an arc in an atomizing air stream. The air flowingacross the arc/wire zone strips off the molten metal, forming a high-velocity spray stream. The process is energy efficient: all inputenergy is used to melt the metal. Spray rate is about 2.3 kg/h/kW(5 lb/h/kW). Substrate temperature can be low because energy input perpound of metal is only about one-eighth that of other spray methods.Conventional plasma: Conventional plasma spraying provides free-plasmatemperatures in the powder heating region of 5500° C. (10,000° F.)with argon plasma, and 4400° C. (8000 F. °) with nitrogen plasma -above the melting point of any known material. To generate the plasma,an inert gas is superheated by passing it through a dc arc. Powderfeedstock is introduced and is carried to the workpiece by the plasmastream. Provisions for cooling or regulation of the spray rate may berequired to maintain substrate temperatures in the 95 to 205° C.(200 to 400° F.) range. Typical spray rate is 0.1 kg/h/kW (0.2 lb/h/kW).Detonation gun: Suspended powder is fed into a 1 m (3 ft) long tubealong with oxygen and fuel gas. A spark ignites the mixture andproduces a controlled explosion. The high temperatures and pressures(1 MPa, 150 psi) that are generated blast the particles out of the endof the tube toward the substrate.High-Velocity OxyFuel: In HVOF spraying, a fuel gas and oxygen areused to create a combustion flame at 2500 to 3100° C.(4500 to 5600 F. °). The combustion takes place at very highchamber pressure (150 psi), exiting through a small-diameter barrelto produce a supersonic gas stream and very high particle velocities.The process results in extremely dense, well-bonded coatings, makingit attractive for many corrosion-resistant applications. Eitherpowder or wire feedstock can be sprayed, at typical rates of 2.3 to14 kg/h (5 to 30 lb/h).Hiqh-energy plasma: The high-energy plasma process providessignificantly higher gas enthalpies and temperatures especially in thepowder heating region, due to a more stable, longer arc and higherpower density in the anode nozzle. The added power (two to three timesthat of conventional plasma) and gas flow (twice as high) providelarger, higher temperature powder injection region and reduced airentrainment. All this leads to improved powder melting, few unmelts,and high particle impact velocity.Vacuum plasma: Vacuum plasma uses a conventional plasma torch in achamber at pressures in the range of 10 to 15 kPa (0.1 to 0.5 atm).At low pressures the plasma is larger in diameter, longer, and hasa higher velocity. The absence of oxygen and the ability to operatewith higher substrate temperatures produces denser, more adherentcoatings having much lower oxide contents.
High quality coatings are “generally” characterized by high adhesion and cohesion strengths, low porosity low oxide inclusions (except for some cases where the phases are small and well dispersed), high hardness, and other properties designed for specific applications such as electrical or magnetic properties, or machinability for finishing.
Particle impact velocity is one of the most important factors in coating quality. One of the main areas of research and innovation in the industry has been the quest for ever higher velocities. Higher velocity impact generally produces denser, harder, and more uniform coatings with less porosity and with higher adhesion and cohesion. Porosity is the largest source of coating failure and is usually indicative of poor coating cohesion and a high degree of unmelted or cold-particle entrapment. High velocity impact forces splats to fill in voids, and the kinetic energy which is converted to heat during the impact reduces the number of unmelted particles, which reduces porosity. Oblique spraying, off perpendicular, should be significantly improved by high velocity, through reduction of shadow porosity effects. In addition, higher velocity tends to produce coatings with less induced stresses.
An aircraft seat is secured by means of a seat rail, which typically includes a central notched groove on the top surface thereof that cooperates with a matching tongue of an interlocking member that secures the seat to the seat rail. During the process of manipulating the seats along the rail to the desired position during installation, reconfiguration, and removal, the groove on the upper surface of the seat rail can get worn. Deep scores, chipped metal, tooling marks, and gouges are typically present. Additionally, vibrations during flight result in constant movement of the seat with the interlocking member against the groove of the seat rail, causing additional wear. Likewise, metal surfaces of the seat rail that are exposed to the environment can corrode due to atmospheric conditions within the plane. Corrosion due to standing water is prevalent. Large amounts of dirt and other organic debris such as food and soft drinks are present in the seat rail groove, providing a constant moist, acidic interface. Corrosion is also observed on all areas of contact between the seat rail and the seat legs where moisture can ingress into mating aluminum surfaces. With the presence of moisture, galvanic effects between the seat rail, interlocking member, and the metal framing to which the seat rail is attached can also cause chemical corrosion along the rail. Generally, the extent of corrosion is proportional to the level of cleanliness of the aircraft interior.
Typically, to reduce wear and corrosion, the seat rails are anodized. Gaps in the corrosion protection, however, include, but are not limited to, all mechanical damage and fastener locations. Corrosion has been found to occur on multiple areas of the seat track and is not always located on corrosion barrier gaps. The seat rails have been painted with an epoxy paint which may contain a corrosion inhibitor well known in the art, such as a chromate-containing corrosion inhibitor. However, it has been found that the coatings previously used for seat rails, in particular aircraft, have not been sufficient to prevent wear within the groove of the seat rail, or to prevent corrosion effects on exposed metal surfaces of the seat rail. Accordingly, the present invention provides a novel coating composition which can be thermally applied to metal surfaces, in particular seat rails for securing aircraft seats, and which has been effective to withstand the wear and corrosion which has plagued these objects.